Many cells Source of electricity
Bulb
Resistor To produce light from electricity
To convert electrical energy into other forms
Variable resistor of energy
To change the value of resistance in a circuit
Open switch To make a circuit open
Closed switch To make a circuit close
Ammeter A To measure electric current in a circuit
Voltmeter To measure voltage in a circuit
Fuse To protect a circuit from excessive heating
Electrical Resistance
Electric current is the rate of flow of electrons. When the electrons move from one part to the
other, they collide with other electrons and with the positive ions present in the conductor. Due
to these collisions, there is some obstruction to the flow of electrons through the conductor.
These collisions are responsible for the slowdown of speed of the electrons. Thus, even as
a conductor conducts electricity, at the same time it offers some obstructions to the flow of
electrons. The property of a conductor by virtue of which it opposes the flow of electrons
through it is called its resistance. The SI Unit of resistance is Ohm (Ω , called omega). Resistance
is denoted by R. It is a scalar quantity.
Combination of resistors
Resistors of different or similar resistance are combined in two ways. They are;
(a) Series combination
(b) Parallel combination
a. Series combination of resistors
The end to end combination of resistors is called a series combination of resistors. In this
combination, there is a flow of the same current but they have different voltage. If three
resistors of resistance R1, R2 and R3 are connected in series, then the total resistance
will be:
R = R1 + R2 + R3
Hence, the total resistance of the circuit in which resistors are connected in series is the
algebraic sum of all the resistors.
+–
R1 R2 R3
Series combination of resistances
Modern Concept Science - 9 143
b. Parallels combination of resistors R1
The type of combination of resistors in which all positive R2
terminals are connected at one point and all negative R3
terminals are connected at another point is called a parallel
combination.
If three resistors of resistance R1, R2 and R3 are connected in V
parallel, then the total resistance will be:
Parallel combination of resistances
1 1 1 1
R = R1 + R2 + R3
Electric Potential and Conventional Direction of Electric Current
When two bodies at different temperatures come in contact, Memory Tips
heat flows from the body at a higher temperature to the body
at a lower temperature by conduction. Such conduction There is 1.5V written on a dry cell.
continues till both the bodies acquire the same temperature. It means that 1.5 joule of work is to
be done by the cell when a charge
Similarly, when two vessels containing water at different of one coulomb moves from the
levels are connected together, then water flows from positive terminal to its negative
the vessel containing water at a higher level to the vessel terminal through the circuit.
containing water at a lower level. Such flow of water
continues till both the vessels have water at the same level.
In the same way, when two unequally charged conductors are joined by a conducting wire,
there is a flow of electrons too. Such flow of electrons is determined by a quantity called the
potential of the conductor. The conductor having excess of electrons is negatively charged
and said to be at a lower potential. The conductor having deficiency of electrons is positively
charged and said to be at higher potential.
FACT WITH REASON
An electric load is connected with two wires for its working, why?
Electric current flows through an electric circuit when there is a potential difference. Out of the two wires
connected to the load, one is at a high potential and another is at a low potential. If the two wires are at the
same potential, electric current does not flow through the load and the load does not work.
Electromotive Force (e.m.f.) and Potential Difference ( p.d.)
Electromotive Force (e.m.f.)
A cell in a circuit does some work for continuous flow of electrons. In order to do work, the
cell utilizes the chemical energy present in it. Electromotive force (e.m.f.) of a cell is defined
as the energy supplied by the cell to drive a unit charge round the whole circuit. The SI Unit
of e.m.f. is volt (V).
Potential Difference (p.d.)
The potential difference between any two points in an electric circuit is defined as the amount
of work done in moving a unit charge from one point to the other point. Potential difference
144 Current Electricity and Magnetism
is abbreviated as p.d. It is a scalar quantity.
Electric potential (V) = Work done = W
Charge q
SI Unit of Potential Difference
The SI Unit of work is joule (J) and that of charge is coulomb (C). So the SI Unit of electric potential
is JC-1, called volt (V).
1 volt
We know that,
1 volt (V) = 1 1 joule (J)
coulomb (C)
Thus, if 1 joule of work is done in moving a unit charge from one point to the other point, then
the potential difference is called 1 volt.
Solved Numerical 7.1
Calculate the work done in a circuit in which electric charge of 12 coulombs passes when a
potential difference of 15V is applied.
Solution: Given,
Potential difference (V) = 15V
Electric charge (q) = 12 C
According to the formula,
Electric potential (V) = W
or, q
W=V×q
or, W = 15 × 12 = 180 J
Thus, the work done is 180J.
Solved Numerical 7.2
How much energy is given to each coulomb of charge passing through a 12 V battery?
Solution: Given,
Potential difference (V) = 12 V
Charge (q) = 1 C
Work done (W) = ?
Now,
Electric potential (V) = W
q
or, W = V × q
or, W = 12 × 1 = 12 J
Thus, the work done in moving each coulomb charge is 12 J.
Modern Concept Science - 9 145
Differences between e.m.f. and p.d.
Electromotive Force (e.m.f.) Potential Difference (p.d.)
1. Electromotive force (e.m.f.) of a cell is 1. The potential difference between any
the energy supplied by the cell to drive two points in an electric circuit is the
a unit charge round the whole circuit. amount of work done in moving a
unit charge from one point to the other
point.
2 It includes the energy required to overcome 2. It does not include the energy required to
the internal resistance of the cell. overcome the internal resistance of the cell.
3. E.m.f. is always greater than p.d. 3. P.d. is always less than e.m.f., when a
current flows through the circuit.
FACT WITH REASON
The potential difference of a source of electric current is always less than its electromotive force (e.m.f.), why?
A source of current like a cell has internal resistance. Some energy of an electric source is wasted in
overcoming this internal resistance. The rest of energy is used to move charge through the external
resistance. It means, the available energy is less than the energy generated. So, the potential difference of a
source of electric current is always less than its e.m.f.
Instruments Used for Measuring Current and Potential Difference
Instruments used for measuring current
a) Ammeter
An ammeter is a device which is used to measure the electric current Ammeter
flowing in a circuit. It is connected in a series in the circuit. It has a very
low resistance. The red colored terminal of ammeter is connected to the
positive side and black colored terminal is connected to the negative
side of the circuit.
FACT WITH REASON
Ammeter is connected in a series in an electric circuit, why?
Ammeter has to measure the actual current flowing in a circuit. Due to small resistance, ammeter does not
affect the total current flowing in the circuit. So ammeter is connected in series in electric circuit.
The ammeter is likely to be burnt out when connected in parallel, why?
An ammeter has very low resistance. If an ammeter is connected in parallel, the resultant resistance of
the circuit decreases and more current passes through it. So the ammeter is likely to be burnt out when
connected in parallel.
b) Galvanometer Galvanometer
A galvanometer is an electrical instrument used to detect the current
in a circuit. It measures low currents. Like ammeter it is also connected
in a series in a circuit. It has a moderate resistance.
146 Current Electricity and Magnetism
Measurement of Potential Difference
The instrument that measures the potential difference across any two points in
a circuit is called a voltmeter. Voltmeter is always connected in parallel to the
two points across which the potential difference is to be measured. It has a red
positive terminal and a black negative terminal.
Differences between Ammeter and Voltmeter Voltmeter
Ammeter Voltmeter
1. Ammeter measures the magnitude of 1. Voltmeter measures the potential
electric current flowing in a circuit. difference between any two points in a
circuit.
2. It has low resistance. 2. It has high resistance.
3. It is connected in series in the circuit. 3. It is connected in parallel across a resistor.
Differences between Volt and Ampere
Volt Ampere
1. It is the SI Unit of potential difference 1. It is the SI Unit of electric current.
(p.d.).
2. Volt = joule/coulomb 2. Ampere = coulomb/ time
3. Its symbol is 'V'. 3. Its symbol is 'A'.
4. It is measured by using a voltmeter. 4. It is measured by using an ammeter.
Ohm's Law
According to Ohm's law, "when physical conditions like temperature, pressure, length of
wire etc. remain constant, then the electric current (I) flowing through a conductor is directly
proportional to the potential difference across its ends."
i.e. I α V (At constant physical conditions)
V = IR
Where, R is a constant called resistance of the conductor. The value of resistance depends on
the nature, length, temperature and area of cross-section of the conductor.
Solved Numerical 7.4
A current of 4A flows through a car headlamp when it is connected to the car battery,
providing a voltage of 12V across the lamp. Find the resistance of the lamp.
Solution: Given,
Potential difference across the lamp (V) = 12V
Electric current (I) = 4A
According to the formula,
R = V = 12 = 3Ω
I 4
The resistance of lamp is 3Ω
Modern Concept Science - 9 147
1 Ohm Resistance
According to Ohm's law
Resistance (R) = Potential difference (V)
Current (I)
When the potential difference (V) = 1 volt, the current (I) = 1 ampere then the resistance of
conductor (R) = 1 ohm
1 ohm = 1 1 volt
ampere
or, 1Ω = 1V
1A
When a current of 1 ampere flows through a conductor by applying a potential difference of 1
volt then the resistance of conductor is one ohm.
Factors Affecting Electric Current Flowing in a Given Conductor
From Ohm's law,
I = V
R
The strength of electric current flowing in a given conductor depends on two factors:
a) Potential difference across the ends of a conductor:
The current is directly proportional to the potential difference. Therefore:
(i) If the potential difference across the ends of a conductor is doubled, the current flowing
through it also gets doubled.
(ii) If the potential difference across the ends of a conductor is halved, the current flowing
through it also gets halved.
(b) Resistance of the conductor
The current is inversely proportional to the resistance. Therefore:
(i) If the resistance is doubled, the current gets halved.
(ii) If the resistance is halved, the current gets doubled.
Factors Affecting Resistance of a Conductor
a) Length of the conductor
From the experimental results, it is found that the 1 metre length of 3 mm wire
resistance of a wire increases with increase in its 2 metre length of 3 mm wire
length. Similarly, on decreasing the length of a wire, 3 metre length of 3 mm wire
its resistance also decreases. So the current is directly
proportional to the resistance of a conductor.
Rαl
148 Current Electricity and Magnetism
FACT WITH REASON
If the length of a conductor is doubled, the resistance also gets doubled. Why?
The resistance is directly proportional to the resistance of a conductor. If the length of a conductor is
doubled, the resistance also gets doubled. If the length of a conductor is halved, its resistance also gets
halved.
b) Cross-sectional area of conductor
The resistance of a conductor is inversely proportional
to its area of cross-section.
R α 1
A
Where, R is the resistance and A is the area of cross- 3 mm wire 6 mm wire
section of the conductor.
FACT WITH REASON
The filament of a lamp is made of a very fine wire, why? αeleAc1 tr.icThene efringye
The resistance of a conductor is inversely proportional to its cross-sectional area. i.e., R
filament of a lamp has a very small cross-sectional area. It offers high resistance to convert
into heat and light energy. So the filament of a lamp is made of a very fine wire.
If the area of cross-section of the conductor is doubled, the resistance gets halved. Why?
The resistance of a conductor is inversely proportional to its area of cross-section. So, if the area of cross-
section of the conductor is doubled, the resistance gets halved. If the area of cross-section of a conductor is
halved, its resistance gets doubled.
The cables that carry a very large current are made of a very thick wire, why?
The resistance of a conductor decreases with increase in its cross-sectional area. The thick cables offer a
very low resistance to the flow of electrons through them. This reduces the heating effect and wires do not
melt while carrying a very large current. It also reduces the loss of electrical energy in the form of heat. So
the cables that carry a very large current are made of a very thick wire.
c) Temperature
The resistance of all pure metals increases with a rise in temperature and decreases with
a decrease in temperature. So, the resistance of a conductor is directly proportional to
the temperature of the conducting wire.
d) Nature of Materials in Circuit
Some materials have a low resistance and some others have a high resistance. For
example, metals like copper, silver, aluminium, etc. have a very low resistance at the
room temperature. Nichrome, constantan, tungsten, etc. have a very high resistance. So,
nichrome is used for making the heating element of heater, electric iron, etc.
Modern Concept Science - 9 149
Electrical Conductance, Resistivity and Conductivity
Electrical Conductance
The reciprocal of resistance is called electrical conductance. It is denoted by G. The unit of
conductance is mho ( ) or ohm-1 or siemen.
Resistivity (or specific resistance)
If 'l' is the length of a wire, 'A' is the cross-sectional area of the wire and 'R' is its resistance, then,
Resistance is directly proportional to the length of the conductor.
R α l …………….(i)
Similarly, resistance is inversely proportional to the area of cross-section of the conductor
A = area
R α A1 …………….(ii) L = length
From relation (i) and (ii), R
R α l
A
or, R = ρAl
When area of cross section = 1m2 and length = 1m.
∴ρ=R
The resistance of a conducting material having unit area of cress section and unit length is
called resistivity. The SI unit of resistivity is ohm-meter or Ωm
Electrical Conductivity
The reciprocal of resistivity is called conductivity. It is denoted by 'σ' . The SI Unit of
conductivity is mho/meter.
Magnetism
Magnets are those substances which can attract magnetic substances like iron, nickel, cobalt,
etc. A freely suspended magnet shows north and south of the earth. The end point of the
magnet which shows the geographical north is the North Pole and the one which shows
the geographical south is the South Pole. Thus, the properties shown by a magnet is called
magnetism. Magnets may be natural magnets like loadstone or may be artificial like bar
magnet, horseshoe magnet, etc. Magnetic substances are those which are attracted by the
magnet and non-magnetic substances are those which are not attracted by the magnet.
Horse shoe-shaped magnet Bar magnet U-shaped magnet
Different types of magnet
150 Current Electricity and Magnetism
Magnetic Compass Magnetic compass
A magnetic compass is an instrument having a small magnetic
needle, which is free to rotate on a pivot at the center of a small box.
The box of magnetic compass has a glass top. It is usually made up
of brass or aluminium. At the base of the box there is a specification
of direction.
Uses of Magnetic Compass
i) Magnetic compassis used to draw magnetic lines of force around a magnet.
ii) Sailors, travelers and navigators use magnetic compass to find the direction.
iii) It is used to test the polarity of the magnet.
Magnetic Field Around a Bar Magnet Magnetic Field
The space around a magnet within which its effect can be experienced
is called the magnetic field. It is a vector quantity. Strong magnets
cover large area and weak magnets cover small area. Magnetic field
can be represented with the help of imaginary magnetic lines of force
drawn around the magnet.
ACTIVITY 1
1. Put a bar magnet on your table and place an iron nail at some distance from
the magnet. Does magnet attract the iron nail?
2. Shift the iron nail closer to the bar magnet. Does it stick to the magnet?
3. It shows that, a bar magnet can attract magnetic substances within its field
only.
Magnetic Lines of Force
If a unit north pole is placed in a magnetic field of a bar magnet, the north pole of the bar
magnet repels it and the south pole of bar magnet attracts it. So it moves from the North Pole
of the bar magnet to the South Pole along a curved path. The path along which a unit north
pole moves in a magnetic field is called a magnetic line of force. Magnetic lines of force are
traced by a small magnetic compass. The pattern of magnetic lines of force is different in
different magnets.
Properties of magnetic lines of force
a) Magnetic lines of force are closed and continuous curves.
b) They never intersect with one another.
c) They are directed from a North Pole to the South Pole outside the magnet and from a
South Pole to the North Pole inside the body of the magnet.
d) The magnetic lines of force are crowded near the poles and they are far apart near the
middle of the magnet.
e) The direction of magnetic field at a point on a magnetic line of force is given by the
tangent at that point.
Modern Concept Science - 9 151
ACTIVITY 2
1. Take a drawing board and fix a chart paper on it with the help of pins.
2. Sprinkle fine powder of iron (iron filings) evenly on the paper.
3. Place a bar magnet in the middle of the chart paper and tap the paper
gently. Does the iron filings rearrange in the form of curves? These curves
represent the magnetic lines of force.
Plotting Magnetic Lines of Force of a Bar Magnet and Neutral Points
We shall draw the magnetic lines of force around a bar magnet depending upon the position
of the bar magnet in the following two ways:
Magnet placed with its north pole pointing towards north of the earth
i) Take a drawing board and fix a white sheet of paper on it. N
ii) Draw two lines in the center of
the paper, i.e., one parallel to
its length (line 1) and another
parallel to its breadth (line 2).
iii) Place a compass at the E
intersection of the two lines.
S
Rotate the drawing board so W
North pole pointing north
that the magnetic needle of
the compass becomes parallel
to the line 2. Mark line 2 as
N-S line with N towards
the geographical north. It
represents the magnetic
meridian. Mark line 1 as E-W
line.
iv) Place a bar magnet on the N-S line with its north pole towards the geographical north
and draw its outline with a sharp pencil.
v) Take a point 0 in front of North Pole of the bar magnet. Place a plotting compass in such
a way that one end of the compass needle coinciding with point zero and mark another
point 1 on the paper at the other end of the compass needle. In the same way plot
number of points like point 2, point 3, point 4 and so on up to the south pole of the bar
magnet. Join all points to obtain a magnetic line of force. Repeat this process to obtain
other magnetic lines of force.
vi) Place the plotting compass on the E-W line. Change the direction of the magnetic needle
and if the needle does not turn at a particular point on the line, that gives the position of
a neutral point (N1).
vii) Similarly, find the neutral point (N2) at the same distance on another side of the E-W
line.
152 Current Electricity and Magnetism
Magnet placed with its south pole pointing towards north of the earth
i) Take a drawing board and fix a white sheet of paper on it. N
ii) Draw two lines in the centre of the paper;
one parallel to its length (line 1) and another
parallel to its breadth (line 2).
iii) Place a compass at the intersection of the E
two lines. Rotate the drawing board so
that the magnetic needle of the compass
becomes parallel to the line 1. Mark line 1 as W
N-S line with N towards the geographical
north. It represents the magnetic meridian.
Mark line 2 as E-W line.
iv) Place a bar magnet on the N-S line with its
south pole towards the geographical north
and draw its outline with a sharp pencil. S
v) Take a point 0 in front of North Pole of the North pole pointing south
magnet. Place a plotting compass in such a
way that one end of the compass needle coinciding with point zero and mark another
point 1 on the paper at the other end of the compass needle. In the same way plot
number of points, i.e., point 2, point 3, point 4 and so on up to the south pole of the bar
magnet. Join all points to obtain a magnetic line of force. Repeat this process to obtain
other magnetic lines of force.
vi) Place the plotting compass on the N-S line. Change the direction of the magnetic needle
and if the needle does not turn at a particular point on the line, that gives the position of
a neutral point (N1).
vii) Similarly, find the neutral point (N2) at the same distance on another side of the N-S line.
Terrestrial Magnetism Rotation axis North
Geographical pole
Our earth behaves like a huge magnet. This fact was first Magnetic field lines
suggested by Dr. William Gilbert 400 years ago. The south
pole of the earth's magnet is towards the geographical north
and the North Pole is towards the geographical South Pole.
Terrestrial magnetism is the study of the earth's magnetism
and its various elements. Magnetic axis
Evidences of Terrestrial Magnetism
Terrestrial magnetism
i) A freely suspended magnetic needle points towards the geographic north and south
direction.
ii) A piece of iron placed inside the earth becomes a magnet.
iii) There exist neutral points.
iv) There occurs angle of dip and angle of declination.
Modern Concept Science - 9 153
FACT WITH REASON
A freely suspended magnetic needle points towards the geographic north and south direction. Why?
A freely suspended magnetic needle points towards the geographic north and south direction because the
free north pole of the magnetic needle is attracted towards South Pole of the earth's magnet. Similarly, the
free south pole of magnetic needle is attracted towards North Pole of the earth's magnet.
Why do neutral points exist?
If we draw the magnetic lines of force due to a bar magnet, then we find a point at which the magnetic field
due to the bar magnet is canceled by the earth's magnetic field. So, neutral points exist.
Earth's Magnetic field
The area where the earth's magnetic influence is felt is called the earth's magnetic field. Like
the magnetic field of a bar magnet, earth's magnetic field is the strongest near the magnetic
north and south poles and the weakest near the equator.
Elements of Earth's Magnetic Field
To determine the magnetic field of the earth at any place, three quantities, namely angle of
dip, angle of declination, and horizontal component of the earth's magnetic field must be
known. These three quantities are called elements of the earth's magnetic field. Here, we will
discuss only two elements, viz. angle of declination and the angle of dip.
Angle of Declination Geographic north
Geographic meridian
In 1492, Columbus observed that a compass needle does
not point to the true north. This is because of the fact that N Angle of declination
the geographic poles and the magnetic poles of the earth S Magnetic meridian
do not coincide. The angle between the magnetic meridian
and the geographical meridian at a place is known as the Angle of declination
declination. The declination is expressed in degrees east
(oE) or degrees west (oW). The angle of declination varies
from place to place on the earth's surface.
Use of Angle of Declination
i) It is used in navigation to find the true geographical direction at a place.
ii) Declination for different places is indicated in every map used by surveyors, mariners
and air pilots. Magnetic needle
Angle of Inclination (Angle of Dip)
In 1576, Robert Norman, a London compass-maker Horizontal H
suspended a compass needle at its center of gravity so plane
that it could rotate in a vertical plane. He found that
it dipped with N pole downwards making an angle of Angle of dip
71.5o. This is the angle of dip at that place. M
Magnetic axis
The magnetic needle suspended at its center of gravity
that is free to rotate in vertical and horizontal planes
Angle of dip
154 Current Electricity and Magnetism
is called dip needle. The angle made by the axis of dip needle with the horizontal line in
magnetic meridian is called angle of inclination or dip. In Kathmandu, the angle of dip is 42o.
Dip at a place is determined by an instrument called a dip circle. In the northern hemisphere
of the earth, the north pole of the magnetic needle dips below the horizontal line whereas in
the southern hemisphere, the south pole of the needle dips below the horizontal line.
Variation of Dip Memory Tips
The strength of the earth's magnetic field is maximum at its The place where dip values are
poles and minimum at its equator. Angle of dip varies from 90° (N-pole down) and 90° (S-pole
place to place on the earth's surface. The angle of dip is zero down) are called the magnetic poles
at the magnetic equator. Its value increases as the magnetic of the earth.
needle is moved towards the north pole or the South Pole.
The value of angle of dip is 900 at the magnetic poles.
Meaning of the Value of an Angle of Inclination (or dip)
a) 0°angle of dip at the magnetic equator : It means that the magnetic field of the earth
at the earth's magnetic equator is parallel to the earth’s surface.
b) 90°angle of dip at the magnetic poles : It means that the magnetic field of the earth at
the earth's magnetic poles is vertical or perpendicular to the earth's surface.
FACT WITH REASON
The angle of dip is 90o at magnetic poles but not at geographical poles, why?
The magnetic north and south poles do not coincide exactly with the geographical poles. The dip needle
points in the direction of magnetic poles. So the angle of dip is 90° at magnetic poles but not at geographical
poles.
c) 42°N angle of dip at Kathmandu : It means that the N-pole of the dip needle makes an
angle of 42o with the horizontal line of the earth.
Use of Angle of Inclination
It is used to find the direction of the earth's magnetic field.
FACT WITH REASON
The angle of dip varies from place to place, why?
Similar to the magnetic field of a permanent bar magnet, earth's magnetic field is the strongest near the
magnetic north and south poles and the weakest near equator. Thus, the direction of the resultant magnetic
field intensity is different at different places on the earth's surface. So the angle of dip varies from place to
place.
i) Neutral Point
When a magnetic compass is placed near a magnet then its needle is under the influence of
two magnetic fields,
Modern Concept Science - 9 155
i) Magnetic field due to the magnet
ii) Magnetic field due to earth's magnet
The horizontal component of the earth's magnetic field (H) is fairly uniform but the magnetic
field due the magnet decreases as we go away from the magnet. Thus, a point around a magnet
at which the field due to the magnet is equal in magnitude but opposite in direction to the
earth's magnetic field is called a neutral point.
FACT WITH REASON
A magnetic compass does not show a particular direction at neutral point, why?
Neutral point is a point around a magnet at which the magnetic field due to the magnet is equal in magnitude
but opposite in direction to the earth's magnetic field. Hence, they cancel each other. The resultant magnetic
field at the neutral point is zero. The needle of a compass placed at the neutral point does not experience a
net force. So, a magnetic compass does not show a particular direction at the neutral point.
ANSWER WRITING SKILL
1. What is one ampere current? How many electrons make one coulomb charge?
Ans: If one coulomb charge flows through a conductor in one second, then the current is called one ampere
current. 6.25 x 1018 electrons make one coulomb charge.
2. Does earth act like a magnet? Define terrestrial magnetism.
Ans: Yes, the earth acts like a magnet. The magnetic properties shown by the earth is called terrestrial
magnetism.
3. Define angle of dip and name a device to measure it.
Ans: The angle made between the dip needle and the horizontal line at a certain place on the earth is called
angle of dip. Angle of dip is measured by Dip circle.
4. What is angle of declination? Write a name of device which is used to measure angle of declination.
Ans: The angle between the geographical meridian and magnetic meridian at a place is called angle of
declination. Angle of declination is measured by a special magnetic compass.
5. Why is conventional flow of current still in practice, despite the fact that it is mistake?
Ans: Conventional flow of current is still in practice because lots of laws on electricity and magnetism are
based on it. It would be tedious to make corrections in all these laws.
6. Differentiate between open circuit and closed circuit
Ans: Differences between open circuit and closed circuit are: Closed circuit
SN Open circuit SN
1 If switch is off in an electric circuit, it is 1 If switch is on in an electric circuit, it is called
called open circuit. closed circuit.
2 Load does not work in open circuit. 2 Load works in closed circuit.
156 Current Electricity and Magnetism
7. Differentiate between resistivity and conductivity.
Ans: Differences between resistivity and conductivity are:
SN Resistivity SN Conductivity
1 Resistivity is the resistance of unit length 1 Reciprocal of resistivity is called
conductor which has unit m2 cross conductivity.
sectional area.
2 The SI unit of resistivity is Ohm-meter 2 The SI unit of conductivity is per ohm-meter
(Ωm). (Ω–1 m–1).
8. If 3 A current is drawn in the electric circuit by connecting 7.5 volt charger, then calculate the resistance
of wire.
Solution:
Current (I) = 3 A
Voltage (V) = 7.5 V
Resistance (R) =?
Using formula,
Resistance (R) = V = 7.5 = 2.5 Ω
I 3
Thus, the resistance of the wire is 2.5 Ω.
9. What charge is delivered if a current of 10A flows for 10 seconds.
Solution:
Current (I) = 10 A
Time (t) = 10 seconds
Electric current through a conductor is given by
I = Q
t
or, Q = I × t = 10 × 10 = 100 C
The total charge delivered is 100 C.
10. How much work is done in moving a charge of 3C across two points having a potential difference 15V?
Solution:
Potential difference (V) = 15V
Electric charge (q) = 3C
According to the formula,
Electric potential = Work done
Charge
or, W = V × q
or, W = 15 × 3 = 45 J
Thus, the work done is 45J.
11. 15A of electric current passes in an electric motor when a potential difference of 4 kV is applied. Find the
resistance in the circuit of electric motor.
Modern Concept Science - 9 157
Solution:
Potential difference across the motor (V) = 4 kV = 4000 V
Electric current (I) = 15A
According to the Ohm's law,
R = V = 4000 = 266.67 Ω
I 15
The resistance of the motor is 266.67 Ω.
12. Angle of dip is 90˚ at poles and 0˚ at equator. Explain the reason behind it.
Ans: Angle of dip is 90˚ at the poles because the magnetic needle of the dip circle is solely pulled by single
pole. As a result, it stays perpendicular to the horizontal surface.
Angle of dip at equator is 0˚ because at equator, magnetic force of both north and the South Pole upon
the deep needle is equal. So, deep needle stays parallel to the horizontal surface.
STEPS EXERCISE
STEP 1
1. Define
a) Current electricity b) Source of electricity
c) Cell d) Electrochemical cell
e) Primary cell f) Secondary cell
g) Photocell h) Solar panel
i) Dynamo j) Hydroelectricity
k) Conductors l) Insulators
m) Charge n) Electric current
o) One ampere p) Electric circuit
q) Open circuit r) Closed circuit
s) Resistance t) Series combination
u) Parallel combination v) Electromotive force
w) Potential difference x) One volt
y) Ammeter z) Galvanometer
aa) Voltmeter ab) Ohm's law
ac) One ohm ad) Conductance
ae) Resistivity af) Magnetic compass
ag) Magnetic field ah) Magnetic line of force
ai) Terrestrial magnetism aj) Angle of declination
ak) Neutral point al) Angle of inclination
158 Current Electricity and Magnetism
2. Very Short Answer Questions
a) Write the SI Unit of the following
i) electric charge ii) e.m.f. iii) p.d.
iv) current v) resistance
b) What is meant by the statement 'potential difference between point A and point
B in an electric circuit is 1 V’?
c) What is the conventional direction of electric current?
d) Name two metals that have a low resistance.
e) State the law that relates the current in a conductor to the potential difference
across its ends.
f) What are the factors on which the resistance of a conductor depends?
STEP 2
3. Short Answer Questions
a) On what factors does the resistance of a conductor depend?
b) How are the flow of electrons in a conductor and the direction of conventional
current related?
c) As the resistance in a circuit increases, what happens to the current?
d) How is the potential difference across a load measured?
e) What will happen to the resistance of a conductor if:
i) the length of the wire is halved?
ii) the area of cross-section of the conductor is doubled?
iii) the temperature of the conductor is increased?
f) A piece of wire is redrawn by pulling it until its length is doubled and cross-
sectional area is halved. Compare the new resistance with the original value.
g) Explain any two evidences of terrestrial magnetism.
h) Write the meaning of the following:
i) 5o E declination
ii) Zero declination
iii) The angle of dip at Kathmandu is 42o N.
4. Differentiate Between
a) Primary cell and secondary cell
b) Open circuit and close circuit
c) Conductors and insulators
Modern Concept Science - 9 159
d) e.m.f. and p.d.
e) Ammeter and voltmeter
f) Angle of dip and angle of declination
5. Give Reason
a) Terminal voltage is less than the e.m.f. of a cell.
b) Ammeter is connected in series and voltmeter is connected in parallel to the
electric load in an electric circuit.
c) The magnetic needle of a compass does not show a particular direction at the
neutral point.
d) The angle of dip varies from place to place.
STEP 3
6. Write down the importance of angle of inclination and angle of declination.
7. Explain the working of a galvanometer.
8. Write the characteristics of magnetic lines of force.
9. Numerical Problems
a) What charge is delivered if
i) a current of 10 A flows for 10 seconds. [Ans: 100C]
ii) a current of 500 mA flows for 10 seconds. [Ans: 5C]
b) How much work is done in moving a charge of 3C across two points having a
potential difference 15V? [Ans: 45J]
c) 48J of work is done in moving a charge of 4C from one terminal of the battery to
another. Find the potential difference of the battery. [Ans: 12V]
d) When a 12V battery is connected to a bulb, the current in the circuit is 2A.
What is the resistance of the bulb? [Ans: 6 Ω ]
e) The resistance for the heater of an electric kettle is 40 Ω. What is the current
flowing through the heater when it is connected to a 240V supply? [Ans: 6A]
10. Draw the Diagram
a) to show how to connect an ammeter and a voltmeter in a circuit.
b) to show the magnetic lines of force around a bar magnet.
STEP 4
11. Derive the formula V= IR, where the terms used have their usual meaning.
12. Derive the formula, ρ = R Al , where the terms used have their usual meaning.
160 Current Electricity and Magnetism
UNIT Estimated teaching periods Theory Practical
8 2
8
Classification of Elements
Syllabus issued by CDC John Dalton
Introduction to classification of elements
Elements and compounds
Structure of atom and electronic configuration
Valence electrons and valency
Radicals and ions
Octet and duplet
Chemical bond (electrovalent bond and covalent bond)
Molecular formula
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
describe and demonstrate the atomic structure of elements and their electronic configuration.
define valency in terms of combining capacity of elements and write molecular formulae of
some common compounds.
describe radicals and ions with examples.
write molecular formulae of some simple compounds.
Key terms and terminologies of the unit
1. Chemistry : The branch of science which deals with the study of matter, its composition and properties is
called Chemistry.
2. Element : An element is the simplest pure form of a substance which cannot be split up into other simpler
substances by any chemical method.
3. Atom : The smallest particle of an element that can take part in chemical reaction without division is
called atom.
4. Compound : Compound is a chemical substance formed by the combination of two or more elements in a
fixed proportion by weight.
5. Molecule : The smallest particle of an element or a compound which is capable of independent existence
is called molecule.
6. Symbol : A symbol is the abbreviation of full name of an element which is represented by one or two
English letters.
7. Atomic number : The total number of protons present in the nucleus of an atom is called atomic number. It is
denoted by Z.
8. Molecular formula :The molecular formula of a molecule is the symbolic representation of the molecule of an
element or a compound in molecular form.
Modern Concept Science - 9 161
9. Atomic mass : The sum of the number of protons and the number of neutrons present in the
nucleus of an atom is called atomic mass.
10. Molecular weight : The molecular weight of a molecule is the sum of atomic weight of all atoms of the molecule.
11. Electronic configuration : The systematic distribution of electrons in different shells of an atom is called
electronic configuration.
12. Valence shell : The outermost shell of an atom from where loss or gain of electrons takes place is
called valence shell.
13. Valence electrons : The total number of electrons which are present in valence shell (outer shell) are
called valence electrons.
14. Valency : The combining capacity of an element or a radical with another element or radical
to form a compound or molecule is called valency.
15. Sub-shells : Each and every main shell contains one or more than one shells which are called
sub-shells. They are denoted by s, p, d and f.
16. Aufbau principle : Aufbau principle states that, "The filling of electrons always occurs from the lower
energy level to higher energy level."
17. Radicals : Radicals are charged atoms or group of atoms having a common charge which act
as a single unit during a chemical reaction.
18. Electropositive radicals : The atoms or group of atoms which have positive charge in them are called
electropositive radicals or basic radicals.
19. Electronegative radicals : The atoms or group of atoms which have negative charge in them are called
electronegative radicals or acidic radicals.
20. Zero valency : Inert gases have complete octet or duplet and show zero valency.
21. Duplet state : The arrangement of two electrons in the K-shell of an atom is called duplet state
22. Duplet rule: Some elements like H, Li, Be try to maintain two electrons in the shell K (last shell)
either by transferring or sharing of electron which is called duplet rule.
23. Octet state : The state of having eight electrons in the valence shell (last shell) of an atom is
called octet state.
24. Octet rule : The tendency of elements by which they try to maintain eight electrons in their
valence shell (last shell) either by transferring or sharing of electrons is called octet rule.
25. Cause of chemical reaction: Losing, gaining or sharing of electrons by an atom to obtain stable electronic
configuration is the main cause of chemical reaction.
26. Chemical bond : The force of attraction by which atoms are held together in a molecule is called
chemical bond.
27. Electrovalent bond : The chemical bond which is formed by the transfer of electron/s from the valence
shell of metal to the valence shell of non-metal is called electrovalent bond.
28. Covalent bond : The chemical bond formed by the sharing of electron pair/s in between two or
more than two non-metal atoms is called covalent bond.
Introduction
The branch of science which deals with the study of matter, its composition and properties is
called chemistry. Anything that occupies space and has mass is called matter, e.g. stone, water,
air, brick, milk, oxygen, etc. All the substances are made of matter.
There are two types of matter. They are pure matter and impure matter. Pure matter includes
elements and compounds whereas impure matter includes homogeneous and heterogeneous
mixtures. Matter is made of tiny particles called atoms.
162 Classification of Elements
Element Memory Tips
An element is the simplest pure form of a substance Altogether 118 elements have been
which cannot be split up into other simpler substances discovered so far but only 92 elements
by any chemical method. An element is made of similar occur in the nature while the rest of the
type of atoms. Hydrogen, carbon, oxygen, sodium, gold, elements are prepared in the laboratory
etc. are some examples of elements. by means of nuclear reactions.
All the substances which are present in our surroundings are either these elements or the
combination of these elements. For example, copper, silver, gold, iron, etc. are the elements
whereas protein, fat, carbon dioxide, water, ammonia, etc. are the combined forms of elements.
Atom 11 p+
12nº
The smallest particle of an element that can take part in
chemical reaction without division is called atom. All 1 p+
the atoms of an element are the same whereas atoms
of different elements are different. For example, all the
atoms of gold in a gold ring are the same but an atom of
gold is different from an atom of silver.
Compound Hydrogen atom Sodium atom
Compound is a chemical substance formed by the combination of two or more elements
in a fixed proportion by weight. Examples: Water (H2O), Sodium chloride (NaCl), Calcium
carbonate (CaCO3), etc. Formation of a compound is a chemical change, so the properties of
a compound differ from its constituent elements. For example, sodium is a toxic metal and
chlorine is a toxic gas but when they combine together they give edible salt (NaCl). In each
compound, the elements are combined by a fixed ratio of their weight. For example, in water
(H2O) molecule, the ratio of weight of hydrogen and oxygen is 1:8.
Molecule
The smallest particle of an element or a compound which is capable of independent existence
is called molecule. Molecule represents the smallest stable condition of matter but it is so small
that it cannot be seen with our naked eyes. In a molecule, there may be one or more than one
atom. These atoms may be similar or dissimilar. For example, helium (He) molecule has one
atom, oxygen molecule (O2) has two similar atoms whereas carbon dioxide (CO2) molecule
has one carbon and two oxygen atoms.
The molecule having only one atom is called monoatomic molecule, e.g. Helium molecule
(He), Neon molecule (Ne), Argon molecule (Ar), etc. The molecule having only two atoms of
the same element is called diatomic molecule, e.g. H2, N2 O2,F2, Cl2, Br2, I2, etc. The molecule
having two or more atoms of different elements is called polyatomic molecule, e.g. NaCl, H2O,
CaCO3, MgSO4, Ca(NO3)2, etc.
Hydrogen (H2) Nitrogen (N2) Oxygen (O2) Fluorine (F2) Chlorine (Cl2) Bromine (Br2) Iodine (I2)
Diatomic molecules
Modern Concept Science - 9 163
Symbol
A symbol is the abbreviation of full name of an element which is represented by one or two
English letters. It is used to make the study easy and fast. If the symbol of an element has one
letter, it is written in capital letter. If it has two letters, the first letter is written in capital and
the second letter in small.
Some examples of elements which are represented by single letter are given below:
Atomic number Elements Symbols Atomic number Elements Symbols
1 Hydrogen H 15 Phosphorus P
5 Boron B 16 Sulphur S
6 Carbon C 23 Vanadium V
7 Nitrogen N 53 Iodine I
8 Oxygen O 92 Uranium U
9 Fluroine F
There are some cases where the names of elements start with the same letter. For example,
hydrogen and helium begin with 'H'. Carbon, chlorine, chromium, cobalt, cadmium, and
calcium start with 'C'. In such conditions, we take the first letter and another significant letter
to symbolize these elements.
Atomic number Elements Symbols Atomic number Elements Symbols
2 Helium He 12 Magnesium Mg
3 Lithium Li 25 Manganese Mn
17 Chlorine Cl 30 Zinc Zn
20 Calcium Ca 40 Zirconium Zr
24 Chromium Cr 22 Titanium Ti
27 Cobalt Co 73 Tantalum Ta
48 Cadmium Cd
There are certain elements whose names are written in English but symbols are used from the
Latin or German name.
English names Latin/German names Symbols
Iron Ferrum (Latin name) Fe
Copper Cuprum (Latin name) Cu
Gold Aurum (Latin name) Au
Silver Argentum (Latin name) Ag
164 Classification of Elements
Mercury Hydrargyrum (Latin name) Hg
Sodium Natrium (Latin name) Na
Potassium Kalium (Latin name) K
Tungsten Wolfram (German name) W
Structure of an Atom 6 protons
+6 neutrons
Atom is the smallest particle of an element. Its diameter
is approximately 10–1°m. In each atom, there are three
fundamental particles. They are electron, proton and neutron. electron
Protons and neutrons are present at the nucleus of an atom proton
whereas electrons are present outside the nucleus in different neutron
shells. Electrons revolve around the nucleus in fixed orbits or
Carbon atom
shells. Structure of an Atom
Structure of a sodium atom Memory Tips
Symbol – Na 1 amu = 1.66 × 10–24 g
Atomic number – 11 11 p+ [amu = atomic mass unit]
No. of protons – 11 12nº 1 coulomb = 6.25 × 1018 esu
No. of neutrons – 12 [esu = electrostatic unit]
Nucleus of an atom is positively charged
No. of electrons – 11 because it has positively charged
Atomic mass – 23 Structure of sodium atom protons. Similarly, an atom is a neutral
particle because it has the same number
Summary of fundamental or subatomic particles of of protons and electrons having equal but
an atom opposite charges.
S.N. Name of the fundamental Symbol Position Mass Charge (in e.s.u)
particles (in a.m.u)
e- Shell -1 e.s.u
1. Electron p+ Nucleus 1/1837 amu +1 e.s.u
n° Nucleus 1 amu 0 e.s.u
2. Proton 1 amu
3. Neutron
Atomic Number
The total number of protons present in the nucleus of an atom is called atomic number. It is
denoted by Z. Chemical properties of an atom depend upon atomic number of that atom.
Atomic number of an atom is also equal to the number of electrons present in a neutral atom.
Atomic number = No. of protons = No. of electrons in a neutral atom
Atomic Mass
The sum of the number of protons and the number of neutrons present in the nucleus of an atom is
called atomic mass. It is denoted by 'A'. Atomic mass of an atom is calculated by the given formula.
Atomic mass = No. of protons + No. of neutrons
Modern Concept Science - 9 165
Electronic Configuration
The systematic distribution of electrons in different Memory Tips
shells of an atom is called electronic configuration. The
atomic shells range from 1 to 7, i.e. K, L, M, N, O, P and The electronic configuration of an
Q. The shell K is the nearest to the nucleus whereas shell atom describes the arrangement of its
Q is the farthest. To explain the systematic distribution electrons around the nucleus.
of electrons in different shells, Bohr and Bury proposed a law which is given below:
1. The maximum number of electrons in each shell is determined by 2n² formula where 'n'
denotes the number of shell in an atom.
For example,
The maximum number of electrons present in shell K,
2n2 = 2 × 12 [ a for shell K, n = 1]
= 2 × 1 × 1 = 2
The maximum number of electrons present in shell L,
2n2 = 2 × 22 [ a for shell L, n = 2]
= 2 × 2 × 2 = 8
Similarly, the maximum number of electrons present in shell M,
2n2 = 2 × 32 [ a for shell M, n = 3]
= 2 × 3 × 3 = 18
This rule is not applicable to 5th, 6th and 7th shells where the maximum number of
electrons is 32,18 and 8 respectively.
2. The maximum number of electrons is not more than 8 and 18 in the outermost shell and
second last shell (i.e., penultimate shell) respectively.
3. It is not necessary to fill the electrons according to the 2n² formula only but a new shell
can be started when there are 8 electrons in second last shell.
4. The shells which are nearer to the nucleus have less energy and the shells far from the
nucleus have more energy.
7
6
5
4
3 Shells or
2 orbits
1
Nucleus 2 8 18 32 32 18 8
P+ n°
K
L
M
N
O
P
Q
166 Classification of Elements
The atomic structures of the first 20 elements are given below:
1p+ 2p+ 3p+ 4p+ 5p+
0n° 2n° 4n° 5n° 6n°
Hydrogen Helium Lithium Beryllium Boron
6p+ 7p+ 8p+ 9p+ 10p+
6n° 7n° 8n° 10n° 10n°
Carbon Nitrogen Oxygen Fluorine Neon
11p+ 12p+ 13p+ 14p+
12n° 12n° 14n° 14n°
Sodium Magnesium Aluminium Silicon
15p+ 16p+ 17p+ 18p+
16n° 16n° 18n° 22n°
Phosphorus Sulphur Chlorine Argon
19p+ 20p+
20n° 20n°
Potassium Calcium
Valence Shell and Valence Electrons
The outermost shell of an atom from where loss or gain of electrons takes place is called
valence shell and the total number of electrons which are present in valence shell (outer
shell) are called valence electrons. For example, valence electrons in sodium, magnesium and
chlorine are 1, 2 and 7 respectively. Valence electrons determine the valency of an atom.
Modern Concept Science - 9 167
Valency
The combining capacity of an element or a radical with another element or radical to form a
compound or molecule is called valency. In the past, valency was defined as the total number of
hydrogen atoms which are combined with an element during chemical combination. For example:
In HCl, the valency number of chlorine is one.
In H2O, the valency number of oxygen is two.
In NH3, the valency number of nitrogen is three.
In CH4, the valency number of carbon is four.
But all compounds do not have hydrogen atoms. So, this concept has been modified and a new
concept is put forth. According to the new concept, "The total number of electrons lost, gained
or shared by an atom during chemical combination is called valency." Valency of sodium is one
because its atom loses one electron during chemical combination and valency of oxygen is two
because its atom gains two electrons during chemical combination. Similarly, the valency of
carbon is four as its atom shares four electrons during chemical combination.
Examples,
1. Find out the valency of aluminium in AlCl3.
In AlCl3, three atoms of chlorine combine with one atom of aluminium. So, valency
of aluminium is three.
2. Find out the valency of hydrogen and phosphate in H3PO4.
In H3PO4, three atoms of hydrogen combine with one phosphate radical. So, the
valency of phosphate is three and that of hydrogen is one.
FACT WITH REASON
The valency of chlorine is one, why?
The valency of chlorine is one as its atoms gains one electron from the atom of other elements during
chemical reaction to gain a stable electronic configuration.
General Idea to Find Out Valency of Some Elements
1. On the basis of the modern periodic table,
i. The valency of an element is equal to the number of group from the first to the
fourth group. For group first, second, third and fourth, the valency is one, two,
three and four respectively.
Group I II III IV
Valency 1 2 3 4
ii. The valency of elements in group fifth, sixth, seventh and eighth is three, two, one
and zero respectively.
Group V VI VII VIII
Valency 3 2 1 0
iii. Valency of zero group elements like He, Ne, Ar, Kr, Xe and Rn is zero.
168 Classification of Elements
2. Valency of a radical is equal to the number of charges present in it. For example:
Radical Valency Radical Valency
Sodium (Na+) 1 Magnesium(Mg++) 2
Carbonate (CO3- -) 2 Phosphate(PO - - -) 3
4
3. In case of transition elements, they have two incomplete outer shells. So, the electrons
of these two shells participate in bonding by showing variable valencies. For example:
Element Lower valency Higher valency
Radical valency Radical valency
Copper Cuprous (Cu+) 1 Cupric(Cu++) 2
Mercury Mercurous(Hg+) 1 Mercuric(Hg++) 2
Gold Aurous(Au+) 1 Auric(Au+++) 3
Iron Ferrous(Fe++) 2 Ferric(Fe+++) 3
Tin Stannous(Sn++) 2 Stannic(Sn++++) 4
Sub-shell Memory Tips
Each and every main shell contains one or more than one The lower valency is ended by suffix
sub-shells which are denoted by s, p, d and f. The main –'ous' and higher valency is ended
shells along with their sub-shells are listed in the given box. by suffix – 'ic'.
Main Shells Sub-shells (orbitals) Shape of s orbital and p orbital
K (n=1) s
L (n=2) s and p
M (n=3) s, p and d
N (n=4) s, p, d and f
The maximum number of electrons that can be accommodated by each sub-shell is given below:
Sub-shells (orbitals) Maximum number of electrons
s (sharp) 2
p (principal) 6
d (diffuse) 10
f (fundamental) 14
The above data shows that the 'K shell' (n = 1) contains only one sub-shell (1s) with maximum
two electrons. 'L shell' (n = 2) contains two sub-shells (2s and 2p) with maximum eight electrons.
'M shell' (n = 3) contains three sub-shells (3s, 3p and 3d) with maximum eighteen electrons. 'N
shell' (n = 4) contains four sub-shells (4s, 4p, 4d and 4f) with maximum thirty two electrons.
Aufbau principle Memory Tips
This principle was given by Wolfgang Pauli and Niels Bohr The last electron present in a sub-shell
in the early 1920s. The different sub-shells of an atom have determines the block of an element in
different energy. Electrons always try to enter into the sub- the modern periodic table.
shell which has less energy.
Modern Concept Science - 9 169
Aufbau principle states that, "The electrons in an atom are
so distributed that they occupy shells in the order of their
increasing energy." It means that the shells having low
energy are filled faster than the shells having high energy
in the following sequence.
1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<
6p<7s<5f<6d<7p
Table: Electronics configurations of some elements on the
basis of sub-shells (s, p, d, and f) are given below:
SN Elements Symbol Atomic Electronic Electronic
number configuration configuration based on Valency
1. Hydrogen based on shells
2. Helium sub-shells
3. Lithium K L MN
4. Beryllium
5. Boron H1 1 1s1 1
6. Carbon He 2
7. Nitrogen Li 3 2 1s2 0
8. Oxygen Be 4
9. Fluorine B5 21 1s2, 2s1 1
10. Neon C6
11. Sodium N7 22 1s2, 2s2 2
12. Magnesium O8
13. Aluminium F9 23 1s2, 2s2 2p1 3
14. Silicon Ne 10
15. Phosphorus Na 11 24 1s2, 2s2 2p2 4
16. Sulphur Mg 12
17. Chlorine Al 13 25 1s2, 2s2 2p3 3
18. Argon Si 14
19. Potassium P 15 26 1s2, 2s2 2p4 2
20. Calcium S 16
Cl 17 27 1s2, 2s2 2p5 1
Ar 18
K 19 28 1s2, 2s2 2p6 0
Ca 20
281 1s2, 2s2 2p6, 3s1 1
282 1s2, 2s2 2p6, 3s2 2
283 1s2, 2s2 2p6, 3s2 3p1 3
284 1s2, 2s2 2p6, 3s2 3p2 4
285 1s2, 2s2 2p6, 3s2 3p3 3, 5
286 1s2, 2s2 2p6, 3s2 3p4 2, 6
287 1s2, 2s2 2p6, 3s2 3p5 1
288 1s2, 2s2 2p6, 3s2 3p6 0
2 8 8 1 1s2, 2s2 2p6, 3s2 3p6, 4s1 1
2 8 8 2 1s2, 2s2 2p6, 3s2 3p6, 4s2 2
Radicals
Radicals are the charged atoms or group of atoms having a common charge which act as a
single unit during a chemical reaction. They have either positive charge or negative charge.
Hence, they do not occur in free form and make different types of compounds.
170 Classification of Elements
FACT WITH REASON
Radicals are highly reactive, why?
Radicals are the charged particles formed by losing or gaining electron(s). They try to pair their
unpaired electrons. So, radicals are highly reactive and least stable.
On the basis of electric charge, radicals are of two types:
1. Electropositive radicals or basic radicals
The atoms or group of atoms which have positive charge in them are called electropositive
radicals or basic radicals. Some examples of electropositive radicals with their valencies are
given below:
Radicals having Radicals having Radicals having Radicals having
valency 1 valency 2 (Bivalent) valency 3 (Trivalent) valency 4
(Monovalent) (Tetravalent)
Beryllium (Be++) Boron (B+++)
Hydrogen (H+) Stannic (Sn++++)
Lithium (Li+) Magnesium (Mg++) Aluminium (Al+++) Plumbic (Pb++++)
Sodium (Na+) Calcium (Ca++) Ferric (Fe+++) Silicon (Si++++)
Potassium (K+) Strontium (Sr++) Auric (Au+++)
Rubedium (Rb+) Barium (Ba++) Chromium (Cr+++)
Caesium (Cs+) Cupric (Cu++) Manganic (Mn+++)
Cuprous (Cu+) Mercuric (Hg++)
Mercurous (Hg+) Stannous (Sn++)
Zinc (Zn++)
Ammonium (NH4+) Nickel (Ni++)
Aurous (Au+) Manganous (Mn++)
Ferrous (Fe++)
2. Electronegative radicals or acidic radicals
The atom or group of atoms which have negative charge in them are called electronegative
radicals or acidic radicals. Some examples of electronegative radicals are given below:
Radicals having valency 1 Radicals having valency 2 Radicals having valency 3
(Monovalent) (Bivalent) (Trivalent)
_ __ ___
Fluoride (F ) Oxide (O ) Nitride (N )
_ __ ___
Chloride (Cl ) Sulphide (S ) Phosphide (P )
_ __ ___
Bromide (Br ) Sulphite (SO3 ) Phosphite (PO3 )
__ ___
Iodide (I –)
Sulphate (SO4 ) Phosphate (PO4 )
_ __
Nitrite (NO2 ) Carbonate (CO3 )
_ __
Nitrate (NO3 ) Zincate (ZnO2 )
Modern Concept Science - 9 171
_ __ __
Cyanide (CN ) Silicate (SiO3 )
__
_
Peroxide (O2 )
Hydroxide (OH ) __
_ Dichromate (Cr2O7 )
__ __
Chlorate (ClO3 )
_ Thiosulphate (S2O3 )
Bisulphate (HSO4 )
_
Bicarbonate (HCO3 )
_
Metaluminate (AlO2 )
Inert Gases
The elements which have eight electrons in their valence shell (except helium) and do not
take part in the chemical reactions are called inert gases. They are kept in the zero group or
VIIIA of the modern periodic table. Inert gases have complete octet or duplet and show zero
valency. Therefore, they are chemically inert and occur in atomic form in gaseous state. The
inert gases with their symbol, atomic number and electronic configuration are given below:
S.N. Name of inert Symbol Atomic K Electronic configuration P
gases number 2 L MNO 8
1. He 2
2. Helium Ne 2 2 8
3. Neon Ar 10 2 88
4. Argon Kr 18 2 8 18 8
5. Krypton Xe 36 2 8 18 18 8
6. Xenon Rn 54 8 18 32 18
Radon 86
On the basis of the above electronic configuration, atomic structures of some inert gases are
given below.
2p+ 10p+ 18p+
2n° 10n° 22n°
Helium Neon Argon
FACT WITH REASON
Inert gases show zero valency, why?
Inert gases have a stable electronic configuration (i.e., complete octet or duplet. ). They do not
undergo chemical reactions and show zero valency.
Ions
Atoms are electrically neutral as they have equal number of protons and electrons having
opposite charges. When an atom loses or gains electron/s, it becomes positively or negatively
172 Classification of Elements
charged. Such type of charged atoms are called ions. So ions are positively or negatively
charged atoms. They are formed by loss or gain of electron/s. Examples: Na+, Mg++, Al+++, K+, Ca+ +,
Fe++, Cu++, Zn++, F–, Cl-, O– –, N– – –, Br–, I–, etc. When an atom loses electron/s from the valence shell,
it becomes positively charged and when an atom gains electron/s, it becomes negatively charged.
Duplet and Octet State
Helium is the first member of inert gases. It has only two electrons and according to the 2n²
rule these two electrons are present in the first shell (K-shell). So, the arrangement of two
electrons in the K-shell of an atom is called duplet state. Inert gases like helium remain in
atomic form.
FACT WITH REASON
A Helium atom remains in atomic form, why?
Since a Helium atom has duplet state, it does not take part in chemical reaction and remains in
atomic form.
Except helium, the other five inert elements have eight electrons in their valence shell. In
other words, they are in stable electronic configuration. The state of having eight electrons
in valence shell (last shell) of an atom is called octet state. The presence of two electrons in
helium (He) and eight electrons in Ne, Ar, Kr, Xe, and Rn is the main cause of stability of these
elements. Hence, they have zero combining capacity (valency).
The elements which have more or less than eight electrons in their last shell are chemically
unstable and they always try to achieve this condition which is called octet rule. So, the
tendency of elements by which they try to maintain eight electrons in their valence shell (last
shell) either by transferring or sharing of electrons is called octet rule. Similarly, some elements
like H, Li, Be try to maintain two electrons in the shell K (last shell) either by transferring or
sharing of electron which is called duplet rule.
Chemical Bond Memory Tips H H
Inert gases have eight electrons Losing, gaining or sharing of Chemical bond
in their valence shell except electrons by an atom to obtain
helium. Due to stable electronic stable electronic configuration is CH
configuration, inert gases are the main cause of chemical reaction.
stable. Other elements which H
do not have duplet or octet state are unstable. Metals have one, Methane
two or three electrons in their valence shell whereas active
non-metals have five, six or seven electrons in their valence shell. Metals try to lose electrons
from the valence shell/s to be stable whereas non-metals try to gain one, two or three electrons
to attain a stable electronic configuration. However, some elements share electron/s to obtain
a stable electronic configuration. Thus, atoms lose, gain or share electrons to obtain stable
electronic configuration. It is the main cause of chemical reaction.
Modern Concept Science - 9 173
When an atom loses electrons, it gains positive charge and when it gains electrons it acquires
negative charge. In between these opposite charges, there is a force of attraction which is called
chemical bond. So, the force of attraction by which atoms are held together in a molecule is
called a chemical bond. For example, in CH4 one atom of carbon and four atoms of hydrogen
are held together by a bond.
There are various types of chemical bonds but in this unit we will discuss electrovalent and
covalent bonds only.
a) Electrovalent bond or Ionic bond
The chemical bond which is formed by the transfer of electron/s from the valence shell of a metal
to the valence shell of a non-metal is called electrovalent bond. The compounds which are
formed by the transfer of electron/s from a metal to a non-metal are called electrovalent
compounds. They contain electrovalent bond/s. For example, NaCl, KCl, CaCl2, MgCl2, CaO, etc.
During electrovalent bonding, metals lose Memory Tips
their electrons and acquire positive charge, i.e. Losing, gaining or sharing of
cation. Similarly, non-metals gain electrons and acquire electrons by an atom to obtain a
negative charge, i.e. anion. In between these opposite charges, stable electronic configuration is
there is a force of attraction which is called electrostatic force the main cause of chemical reaction.
of attraction.
Characteristics of electrovalent or ionic compounds
1. Electrovalent or ionic compounds are generally found in solid state.
2. They have high melting and boiling points. Memory Tips
3. They conduct electricity in molten state or aqueous
In electrovalent compound, the
solution. force of attraction are strong and
4. They dissolve in water. more energy is required to break the
5. They contain metal atom/s in their molecule. force apart. So ionic substances have
Formation of Sodium chloride (NaCl) high melting and boiling points.
Sodium chloride is an electrovalent or ionic compound. It is formed by the transfer of one
electron from sodium atom to chlorine atom. In sodium chloride, sodium is a metal and chlorine
is a non-metal. A sodium atom has one electron in its valence shell, whereas a chlorine atom
has seven electrons in its valence shell. The sodium atom donates its one valence electron to
the valence shell of the chlorine atom. Hence, sodium gains positive charge and chlorine gains
negative charge. There is a force of attraction between two opposite charges which is called
electrovalent bond. This bond keeps Na+ and Cl– together in the form of a molecule, i.e. NaCl.
Sodium atom Na++ Cl– → NaCl
Chlorine atom Sodium chloride molecule
Formation of Sodium chloride molecule
174 Classification of Elements
FACT WITH REASON
NaCl is an ionic compound, why?
During formation of NaCl, there is a transfer of electron from the atom of sodium to the atom of chlorine. It
is the combination of two ions Na+ and Cl- with an ionic bond. So, NaCl is an ionic compound.
Formation of Magnesium chloride (MgCl2)
Magnesium chloride is an electrovalent compound. It is formed by the transfer of two electrons
from one magnesium atom to two chlorine atoms. In magnesium chloride, magnesium is a
metal and chlorine is a non-metal. A magnesium atom has two electrons in its valence shell,
whereas a chlorine atom has seven electrons in its valence shell. During chemical combination,
magnesium atom donates its two electrons to each of chlorine atom. As a result, magnesium
acquires two positive charges and two chlorine atoms gain one negative charge on each. Now,
a chemical bond is formed in between one magnesium and two chlorine atoms.
17p+ 12p+ 17p+
18n° 12n° 18n°
Chlorine atom Magnesium atom Chlorine atom
Mg+++ 2Cl– → MgCl2
Formation of Magnesium chloride molecule
b) Covalent bond
The chemical bond formed by the sharing of electron pair/s in between two or more than two
non-metal atoms is called covalent bond. It is represented by a line (–) in between the bonded
atoms. When one pair of electrons is shared, it is called single covalent bond and represented
by only one line (–). Similarly, when two pairs and three pairs of electrons are shared, they are
called double covalent bond and triple covalent bonds respectively. Double covalent bond is
represented by two lines (=) and triple covalent bond is represented by three lines (≡).
For example:
HH HH
H CCH CC H CC H
HH HH
Ethane (single bond) Ethene (Double bond) Ethyne (Triple bond)
The compounds which have covalent bonds are called covalent compounds. These compounds
are formed by sharing of electron pair/s between non-metallic atoms. Examples: Carbon
dioxide (CO2), Ammonia (NH3), Water (H2O), Methane (CH4), etc. They have generally low
melting and boiling point. There are two types of molecules in covalent compounds. They are:
Modern Concept Science - 9 175
i) Homonuclear molecules
The molecules which have the same type of sharing atoms are called homonuclear
molecules. For example, H2, N2, O2, Cl2, Br2, etc. Memory Tips
ii) Heteronuclear molecules In covalent compounds, the force of
attraction is very weak. So, less energy
The molecules which have different types of sharing is required to break the force apart. So
atoms are called heteronuclear molecules. For covalent substances have low melting
example, CO2, H2O, NH3, CH4, HCl, etc. and boiling points.
Characteristics of covalent compounds
1. Covalent compounds are found in solid, liquid and gaseous state.
2. They have low melting and boiling points.
3. They do not conduct electricity.
4. They are insoluble in water.
5. They do not contain metal atom in their molecules.
Formation of Hydrochloric acid (HCl)
Hydrochloric acid is formed by sharing of one pair of electrons between hydrogen atom and
chlorine atom. In hydrochloric acid, there is one hydrogen atom and one chlorine atom. Hydrogen
atom has only one electron in its shell. So, it requires one more electron to get duplet state. Similarly,
a chlorine atom has seven electrons in its valence shell. So it requires one more electron to get octet
state. Therefore, a hydrogen atom and a chlorine atom share one pair of electrons to get stable
electronic configuration. As a result, hydrogen chloride (HCl) molecule is formed.
1p+ 17p+ 17p+ 1p+
18n° 18n° 0n°
0n° +
Hydrogen atom Chlorine atom Hydrogen chloride molecule
Formation of a hydrogen chloride molecule
Formation of Methane (CH4)
In a methane molecule, there is one carbon atom and four hydrogen atoms. A carbon atom has
four electrons in its valence shell. So it shares its electrons with four electrons of other atoms
to get an octet state. Similarly, hydrogen atom has only one electron in its valence shell. So, it
shares its electrons with one electron of other atoms to get a duplet state. Here, four electrons
of a carbon atom combine with one electron each from four hydrogen atoms to form CH4.
Methane molecule is formed by sharing of four pairs of electrons between one carbon atom
and four hydrogen atoms.
176 Classification of Elements
FACT WITH REASON
CH4 is a covalent compound, why? between an atom of carbon and four
aDtuorminsgoffohrmydartoiogneno.fItCiHs 4th,ethceorme bisinaamtiountuoafltswhoarnionng-omfeetlaelcstwroinths covalent bonds. So, CH4 is a covalent
compound.
011npp°++
011npp°++ 6p+ 011npp°++ H
6n° I
H–C–H
or, I
H
(Methane)
1p+
0n°
Molecular structure of Methane
Differences between electrovalent and covalent compounds
S.N. Electrovalent compounds S.N. Covalent compounds
1. The compounds formed by transfer 1. The compounds formed by sharing of
of electrons between atoms are electron pairs between atoms are called
called electrovalent compounds. covalent compounds.
They cannot conduct electricity.
2. They can conduct electricity in 2.
molten/solution state. They have low melting and boiling point.
3. They have high melting and 3. They do not contain metal atoms in their
boiling point. molecules. Examples: H2O, NH3, CH4, etc.
4. They contain metal atoms in 4.
their molecules. Examples: NaCl,
MgCl2, AlCl3, etc.
Molecular Formula
The molecular formula of a molecule is the symbolic representation of the molecule of an
element or a compound in molecular form. It represents the actual number of atoms of different
elements in a molecule. For example, the molecular formula of sodium chloride is NaCl and
that of water is H2O. It shows that one molecule of sodium chloride (NaCl) consists of one atom
of sodium (Na) and one atom of chlorine (Cl). Similarly, one molecule of water (H2O) consists
of two atoms of hydrogen (2H) and one atom of oxygen (O). Elements like hydrogen, nitrogen,
oxygen, chlorine, bromine and iodine have two atoms in their molecule, viz. H2, N2, O2, Cl2, Br2
and I2 respectively. So, they are called diatomic molecules.
Modern Concept Science - 9 177
Differences between symbol and molecular formula:
S.N. Symbol S.N. Molecular Formula
1. The symbol of an element is the 1. The molecular formula is the symbolic
abbreviation of the full name of representation of a molecule of an element
that element. or a compound.
2. It represents one atom of an 2. It represents one molecule of an element or
element. Examples: H, Na, K, etc. a compound. Example: H2, NaCl, H2SO4, etc.
FACT WITH REASON
An atom of inert gases represents atom as well as molecule, why?
In case of inert gases, i.e. He, Ne, Ar, Kr, Xe and Rn, the single atom represents atom as well
as molecule because they are monoatomic molecules with a stable electronic configuration.
Molecular Weight Memory Tips
The molecular weight of a molecule is the sum of atomic Cl2 indicates one molecule of
weight of all atoms of the molecule. It is calculated by adding chlorine and 2Cl represents two
the atomic weight of the atoms present in a molecule. atoms of chlorine.
i) Molecular weight of Water (H2O) NH3 indicates one molecule of
= H × 2 + O × 1 = 1 × 2 + 16 × 1 = 18 a.m.u. ammonia whereas NH4+ indicates
ammonium radical.
ii) Molecular weight of Calcium carbonate (CaCO3) = Ca × 1 + C × 1 + O × 3
= 40 × 1 + 12 × 1 + 16 × 3
= 100 a.m.u.
iii) Molecular weight of Nitric acid (HNO3) = H × 1 + N × 1 + O × 3 = 1 × 1 + 14 × 1 + 16 × 3
= 1 + 14 + 48
= 63 a.m.u.
iv) Molecular weight of Aluminium sulphate [Al2(SO4)3] = Al × 2 + 3 (S × 1 + O × 4)
= 27 × 2 + 3 (32 × 1 + 16 × 4)
= 54 + 3(96) = 342 a.m.u.
Methods of Writing Molecular Formula
We should follow the following steps to write the correct molecular formula of a molecule.
1. Write the symbol of basic (positive) and acidic (negative) radicals side by side.
2. Write the valency of each radical on upper right corner of each.
3. Exchange the valency of these radicals. Take HCF if it is necessary.
4. Combine radicals with exchanged valency.
178 Classification of Elements
5. If radicals have different atoms, it is enclosed within brackets.
For example, 2. Calcium chloride 3. Aluminium chloride
1. Sodium chloride
11 21 31
Na Cl Ca Cl Al Cl
Na 1 Cl1 Ca1 Cl 2 Al 1 Cl3
NaCl CaCl2 AlCl3
4. Carbon tetrachloride 5. Boron oxide 6. Ammonium sulphate
C4 1 32 1 2
Cl BO NH4 SO4
C1 Cl4 B2 O 3 NH 4 SO 4
CCl4 B2O3 2 1
(NH4)2SO4
7. Calcium sulphate 8. Aluminium nitrate
9. Calcium bicarbonate
2 2 31 Ca2 1
Ca SO4 Al NO3 HCO3
Ca SO4 Al N3O3 Ca HCO 3
2 2 1 1 2
CaSO4 Al(NO3)3 Ca(HCO3)2
Information Obtained form Molecular Formula
1. Molecular formula represents one molecule of a
substance. hydrogen Oxygen
2. It indicates the total number of atoms of the same or 2 atom of 1 atom of
different element/s in each molecule. hydrogen oxygen
3. It indicates percentage composition of each element Information from a molecular formula
present in the compound.
4. The valency or combining capacity of each element can be found from the molecular formula.
i.e. In ammonia molecule (NH3), the valency of nitrogen is three and that of hydrogen is one.
5. We can calculate molecular weight from the molecular formula. For example, the
molecular weight of ammonia (NH3) = N × 1 + H × 3 = 14 × 1 + 1 × 3 = 14 + 3 = 17 a.m.u.
ANSWER WRITING SKILL
1. What are inert gases? Write down their valency?
Ans: Those elements which are naturally stable in octet or duplet form, do not take part in chemical reaction
and found in gaseous state are called inert gases. Valency of inert gas is zero.
Modern Concept Science - 9 179
2. Define duplet and octet rule?
Ans: The tendency of an atom to maintain 2 electrons in the first K-shell of an atom by gaining or losing of
electrons is called duplet rule.
The tendency of an atom to maintain 8 electrons in the valance shell by gaining or losing of electrons is
called octet rule.
3. What is chemical bond? Write down two main types of chemical bonds?
Ans: The force of attraction by which two or more atoms are held together to make a stable molecule is called
chemical bond. Electrovalent and covalent are two main types of chemical bonds.
4. Define electrovalent bond. Give two examples of the compound which have electrovalent (ionic) bond.
Ans: A chemical bond which is formed by the transfer of valence electrons from metal to non-metal is called
electrovalent bond or ionic bond. Examples of compound having electrovalent bond are NaCl, CaCl2, etc.
5. What is a covalent bond? Give two examples of the compounds which have covalent bond.
Ans: The chemical bond which is formed by the sharing of valance electrons between two or more non-metal
atoms is called covalent bond. Examples of the compounds having covalent bond are methane (CH4),
Carbon dioxide (CO2), etc.
6. Write any two differences between Chlorine and chloride.
Ans: Differences between chlorine and chloride are:
S.N. Chlorine (Cl2) S.N. Chloride (Cl–)
i Chlorine is a molecule. i. Chloride is a radical.
ii. Chlorine is electrically neutral (Cl2 ). ii. Chloride is negatively charged (Cl – ).
7. Why is an atom electrically neutral?
Ans: An atom is electrically neutral because the number of positively charged protons is equal to the number
of negatively charged electrons. They cancel each other and make changeless particle.
8. Differentiate between acid radicals and basic radicals.
Ans: Differences between acid radicals and basic radicals are:
S.N. Acid radicals S.N. Basic radicals
i The atoms or groups of atoms which i. The atoms or groups of atoms which
have positive charge in them are called
have negative charge in them are called
electropositive radicals or basic radicals.
electronegative radicals or acidic radicals.
ii. They are formed by the gaining of ii. They are formed by the losing of
electrons. Examples: Cl–, CO3 – – electrons. Examples: H+ , NH4+
9. Differences between NH3 and NH4+.
Ans: Differences between ammonia (NH3) and ammonium ion (NH4+)
S.N. Ammonia (NH3) S.N. Ammonium ion (NH4+)
i Ammonia is a charge less neutral i. Ammonium is a positively charged
molecule. radical.
ii. Ammonia can exist freely in nature. ii. Ammonium cannot stay freely in nature.
180 Classification of Elements
10. Write three characteristics of electrovalent compounds.
Ans: Three characteristics of electrovalent compound are:
i) Electrovalent compound have high melting point and boiling point.
ii) They are soluble in water.
iii) They can conduct electricity in solution or molten form.
11. The atomic number and atomic mass of an element are 11 and 23 respectively. Answer the following
questions on the basis of the given data.
i) Name the element. Will it make electropositive or electronegative radical?
Ans: The name of this element is sodium. It will make electropositive radical because it is a metal
having one electron in its valance shell.
ii) Write down the symbol and valency of this element.
Ans: The symbol of this element is Na and valency is 1.
iii) Calculate the number of electrons, protons and neutrons present in it.
Ans: Number of protons = atomic number = 11
Number of electrons = number of protons = 11
Number of neutrons = atomic mass – number of protons = 23 – 11 = 12
iv) Write sub-shell electronic configuration of this element.
Ans: The sub-shell electronic configuration of Na is 1s2, 2s22p6, 3s1
STEPS EXERCISE
STEP 1
1. Define the following terms with necessary examples if required.
a) Element b) Atom
c) Compound d) Molecule
e) Symbol f) Atomic number
g) Atomic mass h) Electronic configuration
i) Valence shell j) Valence electrons
k) Valency l) Sub-shells
m) Aufbau principle n) Radicals
o) Electropositive radicals p) Electronegative radicals
q) Duplet state r) Octet state
s) Octet rule t) Chemical bond
u) Electrovalent bond v) Covalent bond
w) Molecular formula x) Molecular weight
2. What is meant by monoatomic and diatomic molecules? Give any two examples of
polyatomic molecules.
Modern Concept Science - 9 181
3. Write down the symbol of the given elements:
a) Nitrogen b) Sodium c) Potassium d) Iron
e) Copper f) Chromium g) Gold h) Uranium
4. Write down the atomic number and atomic weight of nitrogen, aluminum and calcium.
5. Define 2n2 rule with one example.
6. Write down the valency of Na, Al, S, Cl and Ar with suitable reason.
7. What are radicals? Mention their types.
8. Write the valency of the given radicals.
a) Carbonate b) Sulphate c) Silicate d) Ammonium
e) Hydroxide f) Phosphate g) Nitrate h) Bisulphate
STEP 2
9. Write any two differences between:
a) Chlorine and Chloride b) Acidic radicals and Basic radicals
c) Duplet state and Octet state d) Atom and Ion
e) Electrovalent compounds and Covalent compounds
10. Give reason:
a) The valency of sodium is one.
b) The valency of chlorine is one.
c) Valency of helium is zero.
d) The valency of oxygen is two.
e) Radicals are very reactive.
f) Inert gases occur in atomic form.
g) NaCl has electrovalent bond but CH4 has covalent bond.
h) An atom is electrically neutral.
i) Neon is called an inert gas.
j) Sodium chloride is called an electrovalent compound.
k) Ammonia is called a covalent compound.
STEP 3
11. Draw the atomic structure of the following atoms:
a) Sodium b) Potassium c) Chlorine d) Aluminium
12. State duplet and duplet rule. Explain in brief with examples.
13. State octet and octet rule. Explain in brief with examples.
14. What is molecular formula? What information do you get from it?
182 Classification of Elements
15. Calculate molecular weight of NH3, H2O, CO2, CH4, H2CO3, CaCO3 and (NH4)2 SO4.
STEP 4
16. What is an electrovalent bond? Explain the formation of electrovalent compound with
one example.
17. What is a covalent bond? Explain the formation of covalent compound with one example.
18. What are electrovalent compounds? Write their characteristics with any three examples.
19. What are covalent compounds? Write their characteristics with any three examples.
20. Write down the molecular formulae of the given compounds by criss-cross method.
a) Aluminium chloride b) Sodium sulphate
c) Calcium carbonate d) Calcium silicate
e) Ammonium nitrate f) Ammonium phosphate
g) Magnesium carbonate h) Calcium phosphate
i) Ferrous carbonate j) Ferric chloride
k) Sodium hydroxide l) Ammonium hydroxide
m) Silver chloride n) Gold chloride
21. Draw the molecular structure of the following molecules:
a) H2 b) O2 c) Cl2 d) NH3
e) HCl f) NaCl g) MgCl2 h) AlCl3
22. Study the given figures and answer the following questions:
11p+ 17p+ 20p+
12n° 18n° 20n°
AB C
a) Name the elements A, B, and C. Write down the valency of each element.
b) Name the compound formed by combination of elements A and B. Also, write the
type of the bond with reason.
c) Name the compound formed by combination of elements C and B. Also, draw the
molecular structure of the resultant compound.
Modern Concept Science - 9 183
UNIT Estimated teaching periods Theory Practical
5 0
9
Chemical Reaction
Syllabus issued by CDC Avogadro
Chemical reaction
Balanced chemical equation
Information obtained from the balanced chemical equation
Exothermic and endothermic reaction
Catalyst, its types and characteristics
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
define physical change and chemical change and differentiate between them.
describe the method of writing chemical equation and write chemical equation.
Key terms and terminologies of the unit
1. Physical change : The temporary and reversible change in which no new substance is
formed is called a physical change.
2. Chemical change : The permanent change in which a new substance having different
properties is formed is called a chemical change.
3. Chemical reaction : The combination, decomposition or exchange that takes place in the
molecules of matter during a chemical change is called chemical reaction.
4. Word equation : The chemical reaction expressed by writing the full names of reactants
and products is called a word equation.
5. Chemical equation : The chemical reaction expressed by writing symbols and molecular
formulae of reactants and products is called chemical equation.
6. Reactants : The chemical substances which take part in a chemical reaction are
called reactants.
7. Products : The chemical substances which are produced after chemical reaction are
called products.
8. Unbalanced chemical equation: The chemical equation in which the total number of atoms of each
element in reactants and products are not equal is called unbalanced or
skeleton chemical equation.
9. Balanced chemical equation : The chemical equation written by balancing the total number of atoms of each
element in reactants and products is called balanced chemical equation.
10. Exothermic reaction : The chemical reaction which releases heat during the chemical change is
called exothermic reaction.
184 Chemical Reaction
11. Endothermic reaction: The chemical reaction which absorbs heat during the chemical change is called
endothermic reaction.
12. Reversible reaction: The chemical reaction which occurs in forward as well as backward directions is
called a reversible reaction.
13. Irreversible reaction: The chemical reaction which occurs only in one direction (forward direction) is
called irreversible reaction.
14. Catalyst: A catalyst is a chemical substance which increases or decreases the rate of
chemical reaction remaining itself chemically unchanged.
15. Positive catalyst : The catalyst which increases the rate of chemical reaction is called positive catalyst.
16. Negative catalyst : The catalyst which decreases the rate of chemical reaction is called negative catalyst.
Introduction
Various types of changes occur in our surroundings as well as within the body of living beings.
For example, rusting of iron, burning of wood, formation of cloud, digestion of food, burning
of fuel, growing of plants and animals, melting of ice, etc. Some of these changes are physical
changes whereas others are chemical changes. In a physical change, new substances are not
formed but new substances having different properties are formed in a chemical change.
Chemical change is a permanent change that occurs as a result of chemical reaction.
Physical Change
The temporary and reversible change in which no new substance is formed is called a physical
change. In a physical change, the chemical composition of the substance does not change but
the physical properties like colour, odour, taste, shape, size, etc. are changed. Physical change
is called a reversible change.
FACT WITH REASON
Physical change is called a reversible change, why?
Physical change is called a reversible change because it occurs in forward as well as back-
ward directions.
on heating on heating
Ice Water Steam
on cooling on cooling
Characteristics of physical change
i. Physical change is a temporary change.
ii. In this change, no new substance is formed.
iii. It is usually a reversible change.
iv. In this change, only physical properties like colour, odour, taste, physical state,
etc. are changed. Memory Tips
Some examples of physical change are given below:
i. Melting of ice or wax Grinding, melting, dissolving, and
ii. Magnetization and demagnetization of iron evaporating are all physical changes.
Modern Concept Science - 9 185
iii. Vaporization of water
iv. Making different objects from wood, soil and paper
v. Formation of a solution
When water is heated, it changes into vapour. It is a physical change. In this change,
new substance is not formed. In this change, water, i.e. liquid state changes into gaseous
state but water and water vapour have similar chemical properties as both contain H2O
molecules as shown in the given figure.
H HH H heat HH HH
OO O O
Water (liquid state) Water vapour
FACT WITH REASON
Melting of ice cubes is a physical chage. Give reason.
Ice cubes melt into water on heating. Ice crystals and water have similar chemical properties
as both contain H2O molecules. Such a change can be reversed back on cooling the water
formed. So, melting of ice cubes is a physical change.
Chemical Change
The permanent change in which new substance having different properties is formed is called
a chemical change. In a chemical change, the physical properties as well as the chemical
composition of the substance are changed. Generally, the product formed during this change
cannot be reversed back to get reactants. So chemical change is also called an irreversible change.
Properties of chemical change Memory Tips
i. Chemical change is a permanent change. Chemical changes take place on the
ii. In a chemical change, new substance is formed. molecular level. It produces a new
iii. It is an irreversible change. substance.
iv. In this change, physical properties and chemical composition are changed.
Some examples of chemical change are given below:
i. Burning of paper and fuel
ii. Rusting of iron
iii. Electrolysis of water
iv. Burning of candle
v. Digestion of food
When water (H2O) is electrolysed, it changes into hydrogen (H2) and oxygen (O2) gas. It is
a chemical change because two new substances having different chemical properties are
formed. This process is shown in the given figure.
H HH H electrolysis HH H H+ O O
OO
Water (H2O) Hydrogen (H2) Oxygen (O2)
186 Chemical Reaction
FACT WITH REASON
Rusting of iron is a chemical change, why?
When iron rusts, iron molecules react with oxygen molecules to make a new compound called
iron oxide. Such a change can not be reversed back. So, rusting of iron is a chemical change.
Differences between Physical change and Chemical change
S.N. Physical change S.N. Chemical change
1. It is a temporary change. 1. It is a permanent change.
2. In this change, no new substance is 2. In this change, new substance is formed.
formed.
3. It is a reversible change. Examples: 3. It is an irreversible change. Examples:
Melting of ice, magnetization of iron, etc. Rusting of iron, burning of paper, etc.
Chemical Reaction
The combination, decomposition or exchange that takes place in the molecules of matter
during a chemical change is called chemical reaction. For example, when hydrogen gas burns
in oxygen, it forms water. It can be expressed as follows:
Hydrogen + Oxygen Burn Water
H2 + O2 Burn H2O
Similarly, when calcium carbonate is heated, it decomposes into calcium oxide and carbon
dioxide gas. It can be expressed as follows:
Calcium carbonate ∆ Calcium oxide + Carbon dioxide
(Reactant) (Product)
CaCO3 ∆ CaO + CO2
A chemical reaction is expressed in word equation and chemical equation.
Word Equation
The chemical reaction expressed by writing the full names of reactants and products is called
a word equation. In a word equation, there are some demerits which are given below:
1. It takes more space and more time.
2. We cannot count the total number of atoms and molecules of reactants and products.
3. We cannot balance the equation.
Examples:
Hydrogen + Oxygen Burn Water
Contact Sodium chloride
Sodium + Chlorine Calcium oxide + Carbon dioxide
Heat Potassium chloride + Oxygen
Calcium carbonate Heat
Potassium chlorate
Modern Concept Science - 9 187
Chemical Equation
The chemical reaction expressed by writing symbols and molecular formulae of reactants and
products is called chemical equation. A chemical equation is more informative than a word equation.
Examples:
2H2 + O2 ∆ 2H2O
2Na + Cl2 2NaCl Chemical 2 H2 (g) O2 (g) 2 H2O (l)
equation 2 molecules of H2 1 molecules of O2 2 molecules of H2O
CaCO3 ∆ CaO + CO2
Molecular
interpretation
2KClO3 ∆ 2KCl + 3O2
Reactants and Products
The chemical substances which take part in a chemical reaction are called reactants. The chemical
substances which are produced after chemical reaction are called products. Reactants are written
on the left side of the arrow whereas products are written on the right side of the arrow.
For example,
HCl + NaOH NaCl + H2O
(Reactant) (Product)
Unbalanced or Skeleton Chemical Equation
The chemical equation in which the total number of atoms of each element in reactants and
products are not equal is called unbalanced or skeleton chemical equation.
For example:
KClO3 ∆ KCl + O2
In above equation, the number of oxygen atoms in reactant
and product sides is not equal so it is called unbalanced
chemical equation. Some more examples of unbalanced
chemical equations are as follows:
Zn + HCl ZnCl2 + H2
KOH + H2SO4 K2SO4 + H2O
H2O + O2
H2O2 MnO2
HCl + Ca(OH)2 CaCl2 + H2O
Balanced Chemical Equation
The chemical equation written by balancing the total number of atoms of each element in
reactants and products is called balanced chemical equation. In this chemical equation, the
number of atoms of each element is equal in reactants and products. It gives more information
than the unbalanced chemical equation. Balanced chemical equation proves the law of
conservation of mass. Above unbalanced chemical equations can be balanced as follows:
188 Chemical Reaction
2KClO3 ∆ 2KCl + 3O2 Memory Tips
MnO2 ZnCl2 + H2
According to Dalton's atomic theory,
Zn + 2HCl atoms can neither be created nor be
2KOH + H2SO4 K2SO4 + 2H2O destroyed and remain intact before
2H2O + O2 and after the chemical reaction. So,
2H2O2 MnO2 every chemical equation should be
2HCl + Ca(OH)2 CaCl2 + 2H2O balanced.
In above equations, the number of atoms of different elements in reactant and product sides
is equal. So, they are called balanced chemical equations.
Methods of Writing Balanced Chemical Equations
Following points should be remembered while balancing the chemical equations:
1. First of all, the chemical change is written correctly in the form of word equation.
For example: Nitrogen + Hydrogen Ammonia
2. The word equation is written correctly in the form of formula equation or chemical
equation.
For example: N2 + H2 NH3
3. The number of atoms of each element are balanced by using suitable coefficient without
changing the molecular formulae of reactants and products.
N2 + 3H2 2NH3
4. The number of atoms in the biggest molecule should
be balanced before balancing the number of hydrogen
and oxygen atoms.
This method of balancing chemical equation is called
hit and trial method.
Some More Examples of Balanced Chemical Equation
1. Word equation : Potassium chlorate heat Potassium chloride + Oxygen
Unbalanced chemical equation: KClO3 ∆ KCl + O2
Balanced chemical equation: 2KClO3 ∆ 2 KCl + 3O2
2. Word equation : Sodium + Chlorine Sodium chloride
Unbalanced formula equation : Na + Cl2 NaCl
Balanced formula equation: 2Na + Cl2 2 NaCl
3. Word equation : Potassium + Oxygen Potassium oxide
Unbalanced formula equation : K + O2 K2O
Balanced formula equation: 4K + O2 2K2O
Modern Concept Science - 9 189
4. Word equation : Magnesium + Oxygen Magnesium oxide
Unbalanced formula equation : Mg + O2 MgO
Balanced formula equation: 2 Mg + O2 2 MgO
5. Word equation : Calcium carbonate ∆ Calcium oxide + Carbon dioxide
Balanced formula equation: CaCO3 ∆ CaO + CO2
6. Word equation : Zinc + Hydrochloric acid Zinc chloride + Hydrogen
Unbalanced formula equation: Zn + HCl ZnCl2 + H2
Balanced formula equation: Zn + 2HCl ZnCl2 + H2
7. Word equation : Calcium chloride + Silver nitrate Calcium nitrate + Silver chloride
Unbalanced formula equation: CaCl2 + AgNO3 Ca (NO3)2 + AgCl
Balanced formula equation: CaCl2 + 2AgNO3 Ca(NO3)2 + 2 AgCl
8. Word equation : Hydrogen peroxide catalyst Water + Oxygen
Unbalanced formula equation: H2O2 MnO2 H2O + O2
Balanced formula equation: 2H2O2 MnO2 2H2O + O2
Information Obtained from a Balanced Chemical Equation
Following pieces of information can be obtained from a balanced chemical equation.
1. The names and symbols of reactants and products
2. The total number of atoms or molecules of reactants and products
3. The ratio of molecular weight of reactant and product molecules
4. The type of chemical reaction
Limitation of a Balanced Chemical Equation
A balanced chemical equation cannot provide information about
1. The physical state of reactants and products
2. Concentration of reactants
3. Conditions required for the reaction like heat, light, pressure, catalyst, etc.
4. The duration of the chemical reaction
5. The rate of chemical reaction.
Modification of Chemical Equation
To make a chemical reaction more informative following modifications are done.
1. The physical state of reactants and products are denoted by 's' for solid,'l' for liquid, 'g'
for gas and 'aq' for aqueous solution.
2. Concentration of reactants are denoted by dil. for dilute and conc. for concentrated.
3. The conditions like temperature, pressure, light, catalyst, etc. are written above or below
the arrow.
4. A double ways arrow ( ⇋ ) is used to represent a reversible reaction and a single way
arrow (→ ) is used to represent an irreversible reaction.
190 Chemical Reaction
5. For endothermic reaction, positive sign (+ ∆) and for exothermic reaction, negative sign
(–∆) is used over the arrow.
Exothermic Reaction
The chemical reaction which releases heat during the chemical change is called exothermic
reaction.
For example,
C + 2H2 CH4 + Heat
CO2 + Heat
C + O2
Ca(OH)2 + Heat
CaO + H2O CO2 + 2H2O + Heat
ZnCl2 + H2 + Heat
CH4 + 2O2
Zn + 2HCl
Endothermic Reaction
The chemical reaction which absorbs heat during the chemical change is called endothermic
reaction.
For example,
N2 + O2 ∆ 2NO
2KClO3 ∆ 2KCl + 3O2↑
CaCO3 ∆ CaO + CO2↑
NaNO2 + NH4Cl ∆ NaCl + 2H2O + N2↑
2NaCl ∆ 2Na + Cl2 ↑
Reversible and Irreversible Reaction
The chemical reaction which occurs in forward as well as backward directions is called a
reversible reaction. Generally, a double ways arrow (⇋) is used to show reversible reaction.
For example, when nitrogen combines with hydrogen, it gives ammonia. But after the
application of low pressure and high temperature, ammonia gets decomposed into hydrogen
and nitrogen.
Nitrogen + Hydrogen Ammonia
N2 + 3H2 2NH3
The chemical reaction which occurs only in one direction (forward direction) is called
irreversible reaction. Generally, a single way arrow (→) is used to show such reactions. For
example, when calcium carbonate is decomposed, it gives calcium oxide and carbon dioxide.
But calcium carbonate cannot be obtained by combining calcium oxide and carbon dioxide.
Calcium carbonate Heat Calcium oxide + Carbon dioxide
CaCO3 ∆ CaO + CO2
Modern Concept Science - 9 191
Catalyst
A catalyst is a chemical substance which increases or decreases the rate of chemical reaction
remaining itself chemically unchanged. There are two types of catalysts.
a) Positive catalyst
The catalyst which increases the rate of chemical reaction is called positive catalyst. For
example, Manganese dioxide (MnO2) acts as a positive catalyst during the decomposition of
hydrogen peroxide.
2H2O2 MnO2 2H2O + O2↑
(catalyst)
b) Negative catalyst
The catalyst which decreases the rate of chemical reaction is called a negative catalyst. For
example, glycerine acts as a negative catalyst and decreases the rate of the given chemical
reaction.
2H2O2 Glycerine 2H2O + O2↑
Characteristics of a catalyst
i) The mass and chemical nature of a catalyst does not change during a chemical change.
ii) A catalyst does not initiate a chemical change.
iii) A catalyst increases or decreases the rate of a chemical reaction.
ANSWER WRITING SKILL
1. Define physical and chemical changes.
Ans: The temporary and reversible change in which no new substance is formed is called a physical change.
The permanent change in which a new substance having different properties is formed is called a
chemical change.
2. What type of change is rusting in an iron rod?
Ans: Rusting of an iron rod is a chemical change.
3. A piece of ice melts into water. What type of change is it?
Ans: Melting of ice into water is a physical change.
4. What is catalyst? Write down its types with one example of each.
Ans: A catalyst is a chemical substance which increases or decreases the rate of chemical reaction remaining
itself chemically unchanged. There are two types of catalysts. They are:
i) Positive catalyst : The catalyst which increases the rate of chemical reaction is called a positive
catalyst. Examples: MnO2, Fe, Cu, etc.
ii) Negative catalyst : The catalyst which decreases the rate of chemical reaction is called negative
catalyst. Examples: glycerine, gypsum, etc.
5. D efine promoter with an example.
Ans: Promoter is a chemical substance which increases the power of catalyst. Example, molybdenum (Mo)
increases the power of iron.
192 Chemical Reaction
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Modern Concept Science and Technology 9
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